Yes, I have had intermittent problems with my 5GHz network, both on the RT-N66U and the RT-N56U. I suspect the cause is the fact that 5GHz radio signals are more easily blocked by obstacles like walls than 2.4GHz signals. That comes from the physics of radio propagation, and there's nothing anybody can do about it.
However, I have not extensively troubleshooted it, and it may in fact be a bug. As far as I recall, I have never had trouble connecting to 5GHz while in the same room as the router. I'm running the stock ASUS firmware, although will be upgrading to merlin soon.
The workaround for me is to just connect my 5GHz devices first to the 2.4GHz network, and then to the 5GHz network. That way, it gives the 5GHz network priority, but is still able to seamlessly fall back to the 2.4GHz network.